Optical sound recording was invented in the beginning of the 20'th century. The sound is recorded by optical means on the film together with the pictures forming the movie. The recorded optical sound track consists of one or two narrow tracks extending along one of the sides of the film next to the sequence of picture frames. The optical sound track may be either intensity or width modulated, the modulation forming an analogue representation of the recorded sound signal. While different standards were used in the beginning of optical sound recording, the prevailing standard is width modulation or variable area tracks, in particular double tracks each with double-sided modulation, providing the best quality and enabling stereo.
A detailed description of optical sound principles, data and standards can be found in the literature, e.g. ISO 2939-1975 (E).
The typical film data and optical design parameters for an optical sound track are shown in Table 1 below:
TABLE 1Total scan area>2.1 mmEach track width0.93-1.03 mm (double track, stereo)Track separation:0.1 mmTypical sound frequencies:20-10.000 HzTypical Film speed:0.5 meters/secondTypical film grain size:3-4 μm
Conventionally, optical sound tracks are recorded using a system with galvanometric blade shutters modulated by the sound and illuminated and imaged onto the film. Modern versions include laser illuminated shutter systems and laser scanning recording systems. Regardless of the technique employed, the bandwidth of optical sound has traditionally been limited to 9-10 kHz particularly when mechanical shutters are used. Modern recording systems can go somewhat higher, perhaps up to 12-15 kHz, though no new standard has been set. On the negative film, the sound track is recorded as black on transparent background.
Originally the advantage of optical sound was the simplicity of the playback. The film is illuminated through a narrow slit so that the common area of the slit and the transparent parts of the sound track determines the amount of light transmitted through the film. A photocell behind the film transforms this modulated light into an electrical signal, which is an analogue electronic representation of the recorded sound signal. The sound may be recorded and played back using noise reduction systems and compression of various types, but this does not change the basic characteristics of the physical sound track, only its dynamics. Thus, optical sound tracks enable utilization of simple, low-cost playback heads on projectors and other equipment.
U.S. Pat. Nos. 4,223,188 and 4,338,684 disclose electronic scanning of the sound track by the detector to obtain width-modulated pulses, which are subsequently demodulated and post-processed in digital-logical circuits.
U.S. Pat. No. 6,351,302 B1 discloses an elaborate digital post-processing system relying on a pulse-width modulated signal by electronic scanning using a CCD array, which is illuminated from the front side.
The simplicity of optical sound is not obtained without noise and distortion. A primary source of noise is dust, dirt, debris and scratches on the film. As a result more noise is generated as the open or transparent areas of the sound track increase. This problem is particularly severe for optical sound track negatives. On negatives, the transparent area on the sound track is largest when the sound signal level is low, and since low-level signals are not nearly as effective as high-level signals in masking noise, the resulting signal-to-noise level may be unacceptable. In particular old movies, which were recorded without modern noise-reduction systems, suffer from this. Re-issuing older films for cinema and transfer to electronic media, CD, DVD and Video put new demands on reproduction of optical sound tracks.